Advances in technology have allowed a variety of electronic devices, including laptop computers, personal digital assistants (“PDAs”), digital audio and compact disk players, to continue to be reduced in size. In addition, the battery chargers used to charge internal batteries used in these devices have also been reduced in size. As battery charger products become smaller, the challenges in managing heat dissipation have increased. Techniques that dynamically limit the amount of power in the battery charging circuit are well known in the art. Also, most battery charging techniques include a fixed parameter, maximum charge time (“MCT”) that is used to terminate the charge operation upon the expiration of a fixed time period.
Charger circuits typically include one or a plurality of a power transistor through which charging currents are regulated by various techniques such as a pulse width modulated (“PWM”) or linear feedback signals applied to the gates thereof. Further, as referenced above, for safety purposes, charger circuits typically include a timer to shut off the charger after the predetermined amount of time, MCT. For example, if a designer is tasked with designing a circuit to charge a one amp-hour battery, the designer must assume that the circuit will take one hour to fully charge a discharged one amp-hour battery when charged at a one amp rate. Assuming the charging circuit is able to dissipate heat during the charge, the designer would set the timer's MCT such that the timer would operate to shut off the charger after about one hour.
However, battery charge times vary based on different conditions. For example, the battery charging current may increase or decrease based upon available power, or the charging current may decrease if the system temperature reaches an excessive level. Alternatively, the user may desire different charging currents based on other criteria such as avoiding peak power costs, or the user may decide to sacrifice battery life for a shorter charging time. In such cases, the charge rate on the battery is dynamically varied through regulation of the charger's power transistor.
While conventional battery charging circuits are operable to dynamically vary the charging process, disadvantageously MCT remains static or fixed. While the designer can take into account the variations in charging currents when setting MCT, MCT is still a fixed parameter which, in conventional circuits, cannot be optimized on the fly based on the charging currents. Typically, MCT of conventional battery chargers are determined based on fixed input voltages, input power levels and operating temperatures. This limits the utility of MCT when the operation of the battery charger is outside the predetermined, fixed parameters.
Inasmuch as conventional battery charging circuits are not operable to dynamically adjust MCT, MCT is typically set for a very long duration and operates solely to insure that the battery is fully charged. Disadvantageously, overly long charging times can be dangerous and tend to defeat the safety purposes of having an MCT. Alternatively, if the maximum charging time is too short, then the battery will not be fully charged resulting in poor charger performance. In battery charging applications where the charging current is not fixed or can vary, MCT must be set for worst case, which it typically based on lowest charging current rate resulting in very long, unsafe MCTs.